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covering a m x n board with dominos

Source: 2021 Dutch IMO TST 3.1

December 28, 2021
combinatoricsgamegame strategywinning strategydominos

Problem Statement

Let mm and nn be natural numbers with mnmn even. Jetze is going to cover an m×nm \times n board (consisting of mm rows and nn columns) with dominoes, so that every domino covers exactly two squares, dominos do not protrude or overlap, and all squares are covered by a domino. Merlin then moves all the dominoe color red or blue on the board. Find the smallest non-negative integer VV (in terms of mm and nn) so that Merlin can always ensure that in each row the number squares covered by a red domino and the number of squares covered by a blue one dominoes are not more than VV, no matter how Jetze covers the board.